Molecular Mechanisms Underlying Meiotic Cohesin Removal – CleaveRec8

In mitosis the correct segregation of the genetic material depends on the timely removal of a ring-shaped, DNA-embracing protein complex, named Cohesin. While phosphorylations in mitotic prophase remove Cohesin from chromosome arms, anaphase only commences when protein phosphatase 2A-protected Cohesin at centromeres is proteolytically cleaved by Separase. In meiosis, two specialized cell divisions without an intervening S-phase generate haploid gametes. The separation of homologous chromosomes first and sister chromatids later requires the stepwise displacement of Cohesin from chromosome arms in meiosis I and from centromeres in meiosis II. Surprisingly, both these divisions require the Separase-dependent cleavage of Rec8, which functionally replaces Scc1 in meiotic Cohesin and needs to be phosphorylated to serve as a substrate for Separase. Whereas the molecular details of stepwise Rec8 removal in the single cell fungus S. cerevisiae are starting to emerge, it remains largely unknown how the localization- and time-specific cleavage of Rec8 is controlled in mammalian meiosis and whether the prophase pathway may contribute to displacement of meiotic Cohesin.

In this collaborative study we will 1) map and functionally analyse the phosphorylations, which render mammalian Rec8 susceptible to cleavage by Separase, and 2) identify the kinases, which are necessary and sufficient to sensitise Rec8 to proteolysis, and characterize phenotypic consequences of their inactivation. A functional Rec8 assay in experimentally more tractable mitotic cells paired with the ability to assess Rec8-cleavage in vitro and in vivo put us in a unique position to achieve these goals. Together with other recent discoveries of ours, this also enables us to 3) unravel the molecular details of rapid Separase inactivation between the two meiotic divisions, 4) clarify why separation of sister chromatids depends on CyclinA in meiosis II and is prematurely triggered by non-degradable forms of this Cyclin but not others, and 5) address whether meiotic cohesin is susceptible to proteolysis-independent dissociation and, if yes, at which position the ring is opened.

We are confident that our mutual efforts will yield important insights into the molecular mechanisms ensuring correct chromosome segregation in mammalian meiosis. In humans, female meiosis is extremely error-prone. Moreover, the error rate increases exponentially with age of the mother. Missegregations in meiosis lead to the generation of aneuploid gametes and, upon fertilization, to aneuploid embryos. Most aneuploidies are not viable and lead to spontaneous fetal abortions, except certain trisomies, such as trisomy 21, which is due to errors in female meiotic divisions in 90% of cases. It has been shown that precocious loss of Cohesin is one reason for the high incidence of oocyte aneuploidies in women closer to menopause. Therefore, we need to understand how Cohesin is removed in meiosis to gain insights into age-related reproductive problems in women.